My brother doesn't listen to my mom at all and she says its because of his adhd, but when his with me and mom is not there he listens. Why is that?

Learned behavior. He has learned that not listening to his mother is "acceptable" behavior. I explain to kids with adhd that their medication does not "make " them behave; they need to make good choices. It's also not an excuse to just not do what they should. His mom will need to work with him to change his behavior rather than just saying "he has adhd".
Nice question. Just because your brother has adhd doesn't mean he can't be a "kid" & act like one. Kids "turn off" their moms regularly, like charley brown's teacher - wawawa- & it has nothing to do with adhd. On the other hand, if his adhd is under treated, his symptoms may emerge at night & present as you describe. This should be discussed with his doctor. Ask if you can fill out a questionnaire on him.
Not the major reason. Adhd may be a factor in how someone responds or behaves, but it should never be an excuse. Your brother may be comfortable with "not listening" to your mom. But if he wants to change, he certainly can do so, and adhd need not stop him.
Associated condition. It is possible that your brother has a condition associated with adhd called opositional defiant disorder. People who suffer from this added problem(also called a comorbidity), often argue with adults and are defiant toward authority figures. There are other criteria associated with this condition so it would take a specialist to make the determination and recommend treatment.